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Structural Steel Frames 321
for a stated period during fires in buildings for the safety of those in the building. Steel,
which is non-combustible and makes no contribution to fire, loses so much of its strength
at a temperature of 550°C that a loaded steel member would begin to deform, twist and
sag and no longer support its load. Because a temperature of 550°C may be reached early
in the development of fires in buildings, regulations may require a casing to structural steel
members to reduce the amount of heat getting to the steel. The larger the section of a
structural steel member, the less it will be affected by heat from fires by absorbing heat
before it loses strength. The greater the mass and the smaller the perimeter of a steel section,
the longer it will be before it reaches a temperature at which it will fail. This is due to the
fact that larger sections will absorb more heat than smaller ones before reaching a critical
temperature.
The traditional method of protecting structural steelwork from damage by fire is to cast
concrete around beams and columns or to build brick or blockwork around columns with
concrete casing to beams. These heavy, bulky and comparatively expensive casings have by
and large been replaced by lightweight systems of fire protection employing sprays, boards,
preformed casing and intumescent coatings. The materials used for fire protection of structural
steelwork may be grouped as:
❏
❏
❏
❏
❏
Spray coatings
Board casings
Preformed casings
Plaster and lath
Concrete, brick or block casings
Spray coatings
A wide range of products is available for application by spraying on the surface of structural
steel sections to provide fire protection. The materials are sprayed on to the surface of the
steel sections so that the finished result is a lightweight coating that takes the profile of the
coated steel, as illustrated in Figure 5.47. This is one of the cheapest methods of providing
Universal
column
Structural
concrete floor
Universal
beam
Sprayed limpet vermiculite or
mineral fibre casing to column
and beam
Figure 5.47 Fire protection of structural steelwork by sprayed limpet casing.